Organopolysiloxane elastomers exhibiting high elongation

ABSTRACT

Organopolysiloxane elastomers exhibiting high toughness and high elongation are obtained upon curing with t-butyl- Beta hydroxyethyl peroxide.

United States Patent Gaylord 1 1 Feb. 11, 1975 1 ORGANOPOLYSILOXANEELASTOMERS 3.234.174 2/1966 Williams 260/465 0 3640.985 2/19 2 StevensEXHIBITING HIGH ELONGATION 3,647,741 3/1972 Hutchinson 260/465 0 [75]inventor: Norman G. Gaylord, New

Providence, NJ.

[73] Assignee: Dart Industries lnc., Los Angeles,

Calif.

[22] Filed: Aug. 20, 1973 [21] Appl. No.: 389,849

[52] US. Cl...... 260/46.5 G, 260/37 SB, 260/610 R [51] Int. Cl. C08f11/04 [58] Field of Search 260/465 G, 610 R, 37 SB [56] References CitedUNITED STATES PATENTS 3,231,542 1/1966 Eisinger et a1. 260/465 G OTHERPUBLICATIONS Kirk-Othmer, Encyclopedia of Chemical Technology, Vol.14,1967, 2nd ed. pp. 814 & 815.

Primary Examiner-Melvyn l. Marquis Attorney, Agent, or Firm-Fred S.Valles; Arthur S. Collins; Margareta LeMaire [57] ABSTRACTOrganopolysiloxane elastomers exhibiting high toughness and highelongation are obtained upon curing with t-butyl-B-hydroxyethylperoxide.

5 Claims, N0 Drawings ORGANOPOLYSILOXANE ELASTOMERS EXHIBITING HIGHELONGATION The present invention relates to organopolysiloxanecompositions exhibiting high elongation upon curing. More particularly,this invention relates to organopolysiloxane compositions obtained bycuring the polysiloxane with t-butyl-B-hydroxyethyl peroxide.

Silicone clastomers are useful in many applications because of theirdesirable physical properties. For ex ample, they exhibit relativelyuniform properties over a wide temperature range, low surface tension, ahigh degree of slip or lubricity, excellent release properties, extremewater repellency, excellent electrical properties over a wide range oftemperature and frequency, inertness and compatability bothphysiologically and in electronic applications, chemical inertness andweather resistance.

It is considered highly desirable for silicone elastomers to exhibit ahigh degree of toughness and elongation; however, most siliconeelastomers that are considered as tough materials usually exhibit hightensile strength coupled with low elongation. In general, siliconcclastomers heretofore available have been characterized by elongationsof from about 50 to about 200 per-cent. Such clastomers have beenprepared by curing organopolysiloxane gums usually employing organicperoxides such as benzoyl peroxide or a dialkyl peroxide such asdi-tert-butyl peroxide as curing agents in admixture with carbonaceousor inorganic fillers. These materials are uniformly blended togethersuch as by milling, mixing and the like. Thereafter, theorganopolysiloxane composition can be cured at pressures ranging fromabout 100 to 2,000 psi or more in combination with temperatures rangingfrom about 100C to 300C or higher. Under such conditions, the timerequired for effecting the desired degree of cure will depend upon suchfactors as the amount of peroxide curing agent utilized, the nature ofthe organopolysiloxane, the type and amount of filler and the ultimateapplication. Subsequently, a post cure treatment is frequently employedin order to eliminate volatile matter carried in with the filler andwith the gum and to eliminate residues resulting from the cure reaction.Post cur ing usually is conducted by heating the cured product at atemperature of about 450500F. for a period of about 24 hours.

The term toughness as employed herein in describing organopolysiloxaneclastomers is defined as the product of the break strength (psi) andelongation of the elastomer. Since toughness is directly proportional toboth the tensile strength and the elongation of the elastomer, it can beseen that toughness has heretofore been imparted to silicone clastomersprincipally through their high tensile strengths. It is desirablehowever, in many applications that a tough elastomer be obtained whichis characterized principally by its high elongation. Elastomers of thistype are especially useful as safety inte rlayers for safety glass,surgical gloves, and the like.

Accordingly, it is an object of the present invention to providesilicone elastomers characterized by high toughness and exhibiting highelongation.

It is another object of the present invention to provide a process forcuring silicone clastomers employing t-butyl-B-hydroxyethyl peroxide asa curing agent.

These as well as other objects are accomplished by the present inventionwhich provides a process for preparing tough silicone clastomersexhibiting high elongation comprising admixing an organopolysiloxane gumcomposed of recurring siloxane groups represented by the formulae:

l {$10} and {SiO-jwherein R is a monovalent hydrocarbon group that isfree of olefinic unsaturation and IR is a monovalent olefinicallyunsaturated hydrocarbon group, said organopolysiloxane containingsufficient olefinic unsaturation to enable crosslinking of the gum toproduce an elastomer when the gum is cured, with an effective cureinitiating amount of t-butyl-fi-hydroxyethyl peroxide, and heating thecurable gum composition to produce a tough elastomer exhibiting highelongation.

The organopolysiloxane gums useful in the present invention arecopolymers that contain recurring silox ane groups represented by theformulae:

R R l l -tSiO-} and {-8101- l R R wherein R is a monovalent hydrocarbongroup that is free of olefinic unsaturation and R is a monovalentolefinically unsaturated hydrocarbon group.

Illustrative of the monovalent olefinically unsatu rated hydrocarbongroups that are represented by an R in the above formulae are thealkenyl groups, for example vinyl, allyl and the like; cycl'oalkenylgroups, for example cyclohexenyl and the like. The preferred monovalentolefinically unsaturated hydrocarbon is the vinyl group.

Illustrative of the monovalent hydrocarbon groups that are free ofolefinic unsaturation and that are represented by R in the formulaeabove are the alkyl groups as, for example, methyl, ethyl, propyl andbutyl, the aryl groupsas, for example, phenyl and the like; the aralkylgroups as, for example, benzyl, phenyl ethyl and the like and thecycloalkyl groups such as cyclohexyl and the like. Illustrative ofuseful organopolysiloxane gums are those composed of methyl vinyl siloxygroups and dimethyl siloxy groups; ethyl vinyl siloxy and dimethylsiloxy groups; methyl vinyl siloxy groups, phenyl vinyl siloxy groupsand dimethyl siloxy groups; methyl vinyl siloxy groups, dimethyl siloxygroups and diphenyl siloxy groups and the like.

Silicon-bonded monovalent olefinically unsaturated hydrocarbon groups,i.e. groups represented by R in the formulae above react to producecrosslinks between the molecules of the starting gum through the agencyof the t-butyl-B-hydroxy ethyl peroxide curing agent of the presentinvention. The relative amount of the unsaturated hydrocarbon groupscontained in the gum is that amount which provides the degree ofcrosslinking necessary to impart elastomeric properties to the curedproduct. Cured products produced from gums that contain an insufficientamount of unsaturation are soft and gummy rather than elastomeric. Curedproducts produced from gums that contain an excessive amount ofunsaturation are hard and brittle rather than elastomeric. Generally,when the amount of siloxane groups containing unsaturated hydrocarbonsrange from about 0.05 part to about 3.0 parts by weight per hundredparts by weight of the total siloxane groups in the gum sufficientcrosslinking can be effected to obtain an elastomeric product.

The specific peroxide curing agent of the present in vention, t-butylZ-hydroxyethyl peroxide, is unique as compared to other conventionallyemployed peroxides for the curing of silicone elastomers. The peroxidehas the following structural formula:

E H C-C-O'-O-CH -CH OH t-Butyl-Z-hydroxyethyl peroxide exhibits a halflife in xylene at a concentration of 0.2 mole at various temperatures asset forth in the Table below:

Half Life t-butyl Z-hydroxyethyl peroxide 'l'cmp (C) Hours l Hour HalfLife Temp. "C I20 I l.0

t-Butyl-Z-hydroxyethyl peroxide is especially advantageous for use inthe present invention in that it does not give rise to halogenated orsimilar harmful residues thereby making it particularly suitable formedical purposes. Its ease of dispersion in the mix insures pin-holefree rubbers which are especially suitable for gasketing materials andelectrical applications. The amount of the peroxide used as a curingagent in this invention is not narrowly critical. The curing agent isusually employed in at least stoichiometric or chemically equivalentamounts, plus when necessary or desirable, a slight excess sufficient tocompensate for any volatilization of the curing agent that might occurduring curing. Generally, useful amounts of the curing agent can varyfrom about 0.1 part by weight to about 4 parts by weight of the peroxideper hundred parts by weight of the organopolysiloxane gum. However, itis preferable to use from about 0.5 part by weight to about part byweight of the peroxide per 100 parts by weight of the organopolysiloxanegum.

A filler can be added to the composition of the present invention.Fillers that are useful in this invention include acidic fillers such asuncoated finely divided silica, basic fillers such as carbon black andalumina and neutral fillers such as quartz, coated finely dividedsilica, calcium carbonate and diatamaceous earth. These fillers impartenhanced tensile strength to the silicone elastomers thereby assistingin the development of a tough elastomer.

Various other additives such as structure additives, pigments, heatstabilizers, antioxidants and the like also can be employed.

The organopolysiloxane compositions of the present invention can beprepared by admixing together, such as by milling, dough mixing, Banburymixing and the like the organopolysiloxane gum, filler, peroxide andother ingredients. The order of addition of the various components isnot critical. It is preferred, however, to add the peroxide to the gumalong with or after the tiller has been added. Thereafter, theorganopolysiloxane composition can be cured and molded at pressures offrom about 1 ,000 to 2,000 psi or more, in combination with temperaturesranging from about C to 300C or higher. A post cure treatment in anaircirculating oven is considered desirable in order to eliminatevolatiles and other residues resulting from the cure reaction. Postcuring can be conducted by heating the cured products at a temperatureof above about 400F for a period of 24 hours.

The following example further defines, describes and compares the toughorganopolysiloxane elastomers of the present invention. Parts andpercentages are by weight unless otherwise indicated.

EXAMPLE I A silicone rubber gum, Silastic 435 Base available from DowCorning, containing 76 wt.% of a polysiloxane having both methyl andvinyl substituent groups on the polymer chain and 24 wt.% of areinforcing silica was mixed on a roll mill for 10 minutes at roomtemperature with Cab-O-Sil MS-7 colloidal silica available from CabotCorporation (5.6 grams per 80 grams rubber). Thereafter, the gum wasadmixed with 0.87 gms (5.9 millimoles) t-butyl-Z-hydroxyethyl peroxidefor 10 minutes. The mixture was cured in a standard rubber mold under20,000 lbs. ram pressure at 350F for 10 minutes (7 half lives) after a 5minute warm-up. The molded slab was then post-cured in an aircirculating oven at 480F for 24 hours.

For comparative purposes 5.9 millimoles of other conventionally employedperoxides were used to cure the silicone rubber under similar conditionsand at temperatures where H) minutes represented 7 half lives. Theamounts of peroxide, the cure temperatures, together with the physicalproperties of the resulting elastomers are shown in the Table below.

MECHANICAL PROPERTIES OF PEROXIDE CURED ORGANOPOLYSILOXANES PeroxideCure Break Ultimate Amount Cure Time Strength Elongation ToughnessPeroxide (gms) Temp(F) (min)" (psi)" (7:)

Example 1 t-butyl-Z-hydroxyethyl peroxide 0.87 350 10 470 740 398.000Comparative Example 1 di-t-butyl peroxide 0.856 340 10 680 180 122.400

do. 2 2,5-dimethyl-2,5-di- 0.86 340 10 780 200 156.000

(t-butylperoxy )hexane do. 3 di-tert-amyl peroxide 1.08 340 10 780 230l79,500

6 MECHANICAL PROPERTIES OF PEROXIDE CURED ORGANOPOLYSILOXANES ContinuedPeroxide Cure Break Ultimate Amount Cure Time Strength ElongationToughness PCl'OXltic (gms)m TemmoF) i jfll p htm 7 (m it) do. 4tcrt'butyl peroxy isopropyl 1.08 280 10 740 l90 l40,700

carbonate do. 5 tcrthutyl perisononunoate 1.37 260 10 660 190 [25.300do. 6 2,4-dichloro benzoyl 1.5 240 5 440 80 35,200

peroxide Gms of peroxide per 80 gms Silastic 435 Base and 56 gms.Cuh-O-Sil MS-7 It can be seen from the above that the use of tertiarybutyl-lhydroxyethyl peroxide in the curing of silicone clastomersprovides an extremely tough elastomer exhibiting a toughness above300,000 and significantly higher elongation than silicone elastomerssimilarly cured using other conventionally employed peroxide curingagents.

Although specific material and conditions are set forth in the aboveexemplary process for preparing an organopolysiloxane elastomer inaccordance with the present invention, these are merely intended asillustrations of the present invention. Various other organopolysiloxaneelastomers, fillers, additives and curing conditions such as thoselisted above may be substituted in the example with similar results.

Other modifications of the present invention will occur to those skilledin the art upon the reading of the present disclosure. These areintended to be included within the scope of this invention.

What is claimed is:

l. A process for preparing heat curable silicone elastomer compositionswhich after complete curing exhibit excellent toughness and highelongation comprising admixing with an organopolysiloxane gum theorganic portion of which is essentially hydrocarbon and 0.05 to 3percent by weight of the organosiloxane units of which contain olefinicunsaturation from about 0.1 part to about 4 parts by weight oft-butyl-B-hydroxy ethyl peroxide per parts by weight of saidorganopolysiloxane gum.

2. The process of claim 1 wherein the t-butyl-B- hydroxy ethyl peroxideis admixed in proportions of'between about 0.5 and about 1.0 part per100 parts by weight of said organopolysiloxane gum.

3. The process of claim 1 wherein most of the hydrocarbon groups in saidorganopolysiloxane gum are methyl groups.

4. The process of claim 1 wherein the olefinic unsaturation in said 0.05to 3 percent by weight fraction of di' organosiloxane units is mostly inthe form of vinyl groups.

5. A process as defined in claim 1 wherein a filler is additionallyadmixed with the gum and the peroxide.

1. A PROCESS FOR PEPARING HEAT CURABLE SILICONE ELASTOMER COMPOSITIONSWHICH AFTER COMPLETE CURING EXHIBIT EXCELLENT TOUGHNESS AND HIGHELOGATION COMPRISING ADMIXING WITH AN ORGANOPOLYSILOXANE GUM THE ORGANICPORTION OF WHICH IS ESSENTIALLY HYDROCARBON AND 0.05 TO 3 PERCENT BYWEIGHT OF THE ORGANOSILOXANE UNITS OF WHICH CONTAIRI OLEFINICUNSATURATION FROM ABOUT 0.1 PART TO ABOUT 4 PARTS BY WEIGHT OFT-BUTYL-BHYDROXY ETHYL PEROXIDE PER 100 PARTS BY WEIGHT OF SAIDORGANOPOLYSILOXANE GUM.
 2. The process of claim 1 wherein the t-butyl-Beta -hydroxy ethyl peroxide is admixed in proportions of between about0.5 and about 1.0 part per 100 parts by weight of saidorganopolysiloxane gum.
 3. The process of claim 1 wherein most of thehydrocarbon groups in said organopolysiloxane gum are methyl groups. 4.The process of claim 1 wherein the olefinic unsaturation in said 0.05 to3 percent by weight fraction of diorganosiloxane units is mostly in theform of vinyl groups.
 5. A process as defined in claim 1 wherein afiller is additionally admixed with the gum and the peroxide.